There is known a fuel pump that includes pump chambers, which sequentially draw fuel and discharge the fuel after compression of the fuel therein. For example, a fuel pump disclosed in JPH06-123288A has an outer gear, an inner gear, a pump housing and an electric motor. The outer gear includes internal teeth. The inner gear includes external teeth and is eccentric to, i.e., is decentered from the outer gear in an eccentric direction. The pump housing rotatably receives the outer gear and the inner gear. The electric motor has a rotatable shaft that is driven to rotate upon energization of the electric motor. Pump chambers are formed between the outer gear and the inner gear. When the outer gear and the inner gear are rotated, a volume of the respective pump chambers is increased and decreased to draw and discharge fuel. A joint member couples between the rotatable shaft and the inner gear. That is, a drive force of the rotatable shaft is transmitted to the inner gear through the joint member.
The joint member and the inner gear discussed above may possibly be configured in a manner shown in FIG. 19. Specifically, FIG. 19 is an enlarged cross sectional view indicating a joint member 160 and an inner gear 120 of a first comparative example. In the drawing, an upward direction along a rotational axis of the inner gear 120 will be also referred to as a first direction, and a downward direction along the rotational axis will be also referred to as a second direction. Furthermore, an upper side of the drawing will be also referred to as a first direction side, and a lower side of the drawing will be also referred to as a second direction side. The inner gear 120 is rotatable in both of a rotational direction Rig and a counter-rotational direction, which are opposite to each other. Legs 164 of the joint member 160 are inserted into insertion holes 127, respectively, of the inner gear 120 in the first direction to transmit the drive force of the rotatable shaft to the inner gear 120 through the joint member 160. FIG. 19 indicates one of the legs 164 of the joint member 160 inserted into the corresponding one of the insertion holes 127 of the inner gear 120. In FIG. 19, a first balance groove 121, which is filled with fuel, is formed in an upper end portion (also referred to as a first direction side end portion) of the inner gear 120, and a second balance groove 153, which is filled with fuel, is formed in a lower end portion (also referred to as a second direction side end portion) of the inner gear 120. A fuel pressure, which is exerted downward in the axial direction by the fuel filled in the first balance groove 121, is balanced with a fuel pressure, which is exerted upward in the axial direction by the fuel filled in the second balance groove 153 to stabilize the orientation of the inner gear 120. Thereby, the inner gear 120 can be rotated in a stable manner.
Inventors of the present application have found that the stable rotation of the inner gear 120 becomes difficult in a case where a relatively large gap space A is present between an upper end surface (also referred to as a first direction side end surface) 161a of the leg 164 of the joint member 160 and a bottom surface (see an imaginary plane 123 of FIG. 19, which is formed by extending of the bottom surface) of the first balance groove 121 of FIG. 19 in the axial direction. Specifically, when the joint member 160 is moved repeatedly by the drive force transmitted from the rotatable shaft in the state where the fuel is filled in the gap space A, a fuel pressure in the gap space A is changed by the movement of the joint member 160. Thereby, the pressure, which is exerted against the inner gear 120 in the upward direction, and the pressure, which is exerted against the inner gear 120 in the downward direction, are unbalanced. Thus, the inner gear 120 is rotated in an unstable manner.
Furthermore, the inventors of the present application have also found the following disadvantage. Specifically, with reference to FIG. 20, which indicates a second comparative example, when an upper end portion (also referred to as a first direction side end portion) 161 of the leg 164 is placed on the first direction side of an upper end (also referred to as a first direction side end) of the first balance groove 121, the leg 164 largely projects from the insertion hole 127 in the first direction. Therefore, the projected portion of the leg 164 may possible contact another member. In such a case, an unnecessary force is applied to the joint member 160, and thereby, the transmission of the drive force from the joint member 160 to the inner gear 120 in the stable manner may become difficult, thereby interfering the stable rotation of the inner gear 120.